1. Field of the Invention
The present invention relates to magneto-optic media which contains a data consistency block, the data consistency block indicating whether or not a directory corruption has occurred during writing of data to the media.
2. Discussion of the Related Art
Magneto-optical disks (MO disks) have the capacity of storing large amounts of data per unit volume. The advantage of an MO disk over a WORM optical disk is that the MO disk can be erased for updating data. The MO disk differs from magnetic tape or magnetic disk media in that the MO disk requires a three-pass write (erase, write and verify) to update the information on the disk. This means that during one revolution of the disk the sectors of the disk which contain the data to be updated must be erased and then on the next revolution of the disk the updated data is written to the affected sectors, on third pass data is read back to verify correctness. This is unlike magnetic tape or disk media which can be updated simply by writing new data over the old data, without an erase pass. It is important with MO disks that there be no intervention between the erase pass and the write pass of new data. If there is a power outage or other interruption after an erase pass, but before the write pass is completed, the data on the MO disk is considered corrupted.
If data on an MO disk is corrupted, it is important that the user be notified. After there is knowledge that there is corrupt data on the MO disk it is important that there be a positive method of recovery. Several schemes have been proposed to detect and recover corrupted data on an MO disk. One scheme is to use intent and update counters which are written on the MO disk. When a data update is made the intent counter is incremented to its next highest integer. Assume for example that each of the intent and update counters indicate the integer 25. Preparatory to an update a three-pass write is issued to increment the intent counter to the next highest integer (26). Next, a three-pass write is issued to the affected sectors on the MO disk to perform the update. After the update has been verified, the update counter is incremented to the next highest integer, (26), by another three-pass write. This scheme will indicate to subsequent users that the last update to the MO disk was successful. When the updated data is next accessed the intent and update counters are first inspected. In the example both counters contain 26, which indicates that the data on the MO disk is not corrupted. However, should the contents of the intent and update counters be unequal, this will indicate that the last update was not successful and that the data has been corrupted. Recovery measures will then be taken including accessing the updated data and issuing a three-pass write to the affected sectors on the MO disk, provided the updated data has been stored to some stable memory device.
The problem with the above scheme for detecting corrupted data on an MO disk is that it requires three three-pass writes in addition to the work to back up the updated data. First, there is a three-pass write to the intent counter, then there is a three-pass write to the affected sectors on the MO disk, and then there is a three-pass write to the update counter. When a large amount of updates are made, this requires time to seek between the intent and update counters and time for the MO disk to make the number of revolutions for three three-pass writes. When performing updates in an MO disk library, the accumulation of the above time periods becomes quite significant. In addition storing of the updated data also needs to be accomplished
The standard method of providing this type of protection on magnetic devices is to write the updated data to the disk itself, perform the update, then issue another write to indicate that the disk is consistent. Direct application of this technique to MO devices also results in a minimum of three three-pass writes to the media. The present invention replaces two of the three-pass writes with one two-pass write (a write and verify pass) and one one-pass erase.
Other approaches involve changing the directory structure or keeping it in duplicate. These approaches have negative performance and/or media utilization consequences.